Such disc brakes which in practice are mounted in pairs on either side of the brake disc define safety brakes. The actuating unit which may be hydraulically or magnetically operated normally holds the brake shoe out of contact with the brake disc and when necessary the motive power of the actuating unit is cut off and the resilient control means which also effectively acts on the brake shoe positively urges the brake shoe against the brake disc.
Such brakes are therefore typically used for industrial purposes, for example for travelling cranes, winches, drums and conveyors, such as disclosed in French Pat. No. 1,412,483.
One of the problems to be overcome with brakes of this type resides in the circumferential drag forces imparted to the brake shoe owing to its contact with the brake disc. This circumferential force must be withstood by the fixed support and therefore transmitted to the fixed support by the brake shoe while the brake shod admits of axial displacement so it is maintained against the brake disc even when the friction facing is worn.
Heretofore the contact surfaces between the brake shoe and the fixed support have both been cylindrical which is the case in particular with the above mentioned French patent.
Because of the circumferential drag force imparted to the brake shoe it has the tendency to move out of alignment with the axis of the bore in which it is slidably mounted so that with the corresponding surfaces being cylindrical there is always somewhat of a likelihood of brake shoe becoming jammed in the bore which obviously jeopardizes axial displacement of the brake shoe.
Further considerable stresses which are liable to damage the contact surfaces then develop between the brake shoe the fixed support in line with the corresponding contact surfaces.
These risks of jamming and damages are all the greater as the vibration, however small they may be, often add to the circumferential force. The brake shoe is inevitably subjected to such vibrations owing the possible defects in the geometrical configuration of the brake disc with which it is in contact.